1. Field of the Invention
This invention relates to an agent for protecting the surface of a lithographic printing plate, especially to an emulsion type plate protective agent.
2. Description of the Prior Art
The final step of the process of making a lithographic printing plate is generally referred to as a gumming step since in this step the plate is coated with a gumming solution to protect the non-image areas (which retain water and repel greasy printing ink). While the primary purpose of gumming is to desensitize or increase the hydrophilicity of the non-image area of the lithographic printing plate it has other purposes. Namely, to prevent possible deterioration of the lithographic printing plate during the period between making the plate and printing or during the period when printing is interrupted; to prevent the non-image area from becoming receptive to printing ink due to sebum on fingers and other foreign matter during handling of the lithographic printing plate such as when it is set on a printing machine; and to prevent development of scratches and other flaws on the non-image area or image-area (which repels water and accepts printing ink) during handling of the plate.
Among conventional gumming solutions, the most effective is an emulsion type gumming solution which comprises an oil phase having dissolved in an organic solvent a lipophilic substance soluble in the solvent plus an emulsifying surfactant and an aqueous phase having water soluble dextrin dissolved therein. When this type of gumming solution is used to gum a plate for lithographic printing, the image area is protected by the lipophilic substance in the oil phase whereas the non-image area is protected by the water soluble dextrin in the aqueous phase. As a result, the lipophilicity of the image area decreases only slightly even if it is in contact with the water soluble dextrin.
However, this emulsion type gumming solution is still unable to prevent a decrease in the lipophilicity of the image area completely.